The present invention relates to a control method and a control system for an automatic transmission.
A motor vehicle with a manual transmission offers better fuel economy as compared with a motor vehicle mounted with a transmission using a torque converter. It is, however, difficult in such a motor vehicle as that with the manual transmission to manipulate a clutch and an accelerator in a coordinated manner when getting the motor vehicle started. If the coordinated operations of the clutch and the accelerator do not go well when the vehicle is started, a large shock occurs as the clutch is engaged. If there is a short supply of a clutch pressure, the engine speed rises sharply, which is commonly referred to as a “revving up phenomenon.” If the clutch is quickly engaged when the engine speed is yet to reach a sufficient level, or when the vehicle is started on an uphill slope, the engine can stall.
To solve these problems, a system that automates the clutch and a gearshift using a manual transmission mechanism has been developed. The system is what is called an automated manual transmission, or automated MT.
The conventional automated MT has the disadvantage that there is an interruption of a driving torque occurring from release and engagement operations of the clutch during control for gearshifts. This can at times give passengers of the vehicle a sense of discomfort.
A vehicle provided with an automatic transmission has been proposed to avoid the torque interruption during the gearshift. A known approach to the problem is an assist clutch serving as a transmitting torque variable mechanism provided for the conventional automated MT (see, for example, Japanese Patent No. 2703169). The assist clutch provides control, during a gearshift, for synchronizing rotations and transmitting torque properly for the gearshift. In a vehicle such as that described above, an assist clutch torque release control is provided. In the assist clutch torque release control, after the gearshift to a new meshing gearing has been completed, torque transmitted by the assist clutch is reduced, while torque transmitted by the new meshing gearing after the gearshift is increased.
When the torque transmission through the assist clutch is changed to the torque transmission through the meshing gearing after the gearshift, torque vibration in a transmission output can occur due to a torque step depending on how the assist clutch is controlled. The assist clutch, on the other hand, transmits torque through slippage. This presents a durability problem arising from a quantity of heat generated.